Each side blamed the other for starting clashes that broke out during the night and intensified before dawn and spread to several locations, with one Thai soldier and four Cambodian civilians killed, according to officials.
Cambodia accused Thailand of "inhumane and brutal acts" of aggression, stressing it had not retaliated, while Bangkok said it carried out air strikes on military targets after its neighbour mobilised heavy weaponry and repositioned combat units.
"The objective of the army is to cripple Cambodia's military capability for a long time to come, for the safety of our children and grandchildren," Thai army chief of staff General Chaipruak Doungprapat said, according to the military.
The fighting was the fiercest since a five-day exchange of rockets and heavy artillery in July that marked their heaviest clashes in recent history, when at least 48 people were killed and 300,000 displaced before Trump intervened to broker a ceasefire.
Tensions have simmered since Thailand last month suspended de-escalation measures that were agreed at a summit in Trump's presence, after a Thai soldier was maimed by a landmine that Bangkok said was newly laid by Cambodia.
Some of the mines that have wounded seven Thai soldiers since July were likely newly laid, Reuters reported in October, based on expert analysis of material shared by Thailand's military.
Cambodia has denied laying the mines and Thailand has said it will not implement the ceasefire terms until Cambodia apologises.
Thai Prime Minister Anutin Charnvirakul on Monday said his government would do whatever necessary to protect its territorial integrity and would not engage in dialogue with Cambodia.
"There will be no talks. If the fighting is to end, (Cambodia) must do what Thailand has set," he said, without elaborating.
Cambodia's defence ministry said its forces came under sustained attack but were committed to the ceasefire and did not retaliate.
"Cambodia calls on the international community to strongly condemn Thailand's violations ... as well as demands that Thailand take full responsibility for such brazen acts of aggression," it said in a statement.
Thailand's army said Cambodia used drones to drop bombs on Thai bases and fired truck-mounted BM-21 rockets towards civilian areas.
A Thai military official told Reuters targets of air strikes included long-range Chinese-made rockets.
Malaysian Prime Minister Anwar Ibrahim, chair of the regional bloc ASEAN who helped Trump broker the truce, called for calm and for communication channels to stay open.
"The renewed fighting risks unravelling the careful work that has gone into stabilising relations," Anwar said in an X post.
UN Secretary-General Antonio Guterres urged Thailand and Cambodia to exercise restraint and avoid further escalation, said his spokesperson, adding: "The United Nations stands ready to support all efforts aimed at promoting peace, stability,Â
Cambodia's former longtime leader Hun Sen, the influential father of current premier Hun Manet, said Thailand's military was seeking to provoke a retaliatory response.
"All frontline forces must remain patient because the aggressors have been firing all kinds of weapons," he said on Facebook.
Thailand evacuated 438,000 civilians across five border provinces and authorities in Cambodia said hundreds of thousands of people had been moved to safety. Thailand's army said 18 soldiers were wounded and Cambodia's government reported nine civilians injured.
